Generally, a hammer drill is structured such that a piston is driven by a motor to reciprocate back and forth on the axis of a top end tool to thereby vary the air pressure of an air chamber formed between the piston and a hammering member; and thus, variations in the air pressure (air spring) are used to allow the hammering member to generate its driving motion. And, the hammer drill transmits the driving force of the hammering member through an intermediate member to the top end tool and also transmits the rotation of the motor through a reduction mechanism such as gears to the top end tool, thereby drilling a hole in concrete or the like. (For example, JP-A-61-164785).
Here, in an operation to drill a hole in the surface of the ceiling, which is one of the main operations of the hammer drill, in order to be able to enhance the efficiency of the drilling operation, the handling of the main body of the tool must be easy and the time necessary for the operation must be short. This demands that the hammer drill is small in size and light in weight as well as is capable of drilling the hole at high speeds.
Generally, to develop a tool which can realize high speed hole drilling, the driving energy of the tool per driving may be increased or the number of rotations of a motor may be increased to thereby increase the number of times of hammering. However, when increasing the driving energy per driving, the mass of the hammering member must be increased and also a driving mechanism portion for driving the hammering member must be increased in size. Accordingly, the size of the main body of the tool is increased, thereby to impair the efficiency of the operation. In view of this, preferably, there may be selected a method which increases the number of rotations of the motor and thus increases the number of times of hammering to thereby increase the hole drilling speed.
However, when the hole drilling speed is increased by increasing the number of times of hammering, at a certain number of times of hammering, the hammering member becomes unable to follow the piston and thus the driving force of the hammering member becomes weaker accordingly, whereby the hole drilling speed is lowered. As described above, in linking with the rotation of the motor, the piston is allowed to reciprocate and thus vary the air pressure of the air chamber, whereby the hammering member is allowed to generate its driving motion. That is, when the reciprocating motion of the piston is too fast, the hammering member is not able to follow the variations in the air pressure, whereby the driving motion of the hammering member is disturbed and thus the driving force of the hammering member is weakened. As the number of times of hammering where a speed balance of the hammering member with respect to the piston starts to lose, there is known the limit number of times of hammering. With the limit number of times of hammering as the boundary, the hole drilling speed is lowered suddenly and heavily, thereby generating so called poor driving.
Thus, the number of rotations of the motor may be lowered in such a manner that the number of times of hammering can be prevented from reaching the limit number of times of hammering. In view of this, there is sold on the market a hammering tool in which, with the variations of the characteristics of motors between the motors taken into consideration, the number of rotations of a motor is set low in order that the number of times of hammering of the hammering tool can be prevented from reaching the limit number of times of hammering.
However, when the number of rotations of the motor is set low so that the number of times of hammering of the hammering tool can be prevented from reaching the limit number of times of hammering, it is not sufficient to take only the variations in the motor characteristics into consideration. That is, the restitution coefficients of colliding parts differ depending on the strength of the concrete, in which a hole is to be drilled, as well as on the mass and shape of the top end tool. Such restitution coefficients are greatly involved with the driving motion of the hammering member; that is, the limit number of times of hammering is different due to the restitution coefficients. Therefore, when these variations are also taken into consideration, the number of times of hammering of the hammering tool when it is designed must be set greatly lower than the limit number of times of hammering.
Also, for a hammering tool of a charging type, during operation, the battery voltage of the motor of the tool decreases and thus the number of rotations of the motor decreases, whereby the number of times of hammering of the tool is lowered greatly from the limit number of times of hammering and thus the driving efficiency of the hammering member is also lowered.